I admit since I don't speak German I can't evaluate that, but I can try to make sense of the following in English as a particularly disturbing example...

"Again abandoned in order to return to the showing mode, it proves the input mode as amazingly tricky. To expect it would be that to everyone mouse-click outside of the text field the editing mode terminated."

After rereading some things that made no sense they began to make small sense. Some, like the above text, make little sense at all. Frankly I'd rather be standing on a street corner in Frankfurt exchanging hand signals and pantomime than reading this as my mind bobs up and down trying to assemble it into a rational sentence structure. At least we could communicate on where to get a good pilsner. ;-)

"To exit the edit mode again to return to the view mode is amazingly tricky. We would expect that any click outside of the text-field terminated the edit mode. Actually one must confirm changes with the [Return] key or cancel with the [ESC] key. Clicking into the help window or a double click on another structure component exits the input mode as well, although in these cases Umbrello loses any changes made."

Hey! What is everyone complaining about the google translation?
IMO it's pretty impressive what the google translation sometimes
can achieve today. Don't you think you are expecting a bit to much
if such a complicated article is to be translated automatically??
This article is especially difficult because it heavily uses the ability of
the German language to put many parts in arbitary order into a sentence
and then link them with grammar and sometimes only the order gives
the meaning. You can do that in English. Therefore, the longer a sentence
the more difficult is it for a computer to translate properly because it only
translates words. In cannot "translate" a different word order into something.
Anyway, I made a quick translation which isn't perfect because I did it in
30 minutes and have no idea about UML but I hope it's easier to understand
than the google version:

Umbrello is the best tool for UML on Linux. I've tried to install Rational Rose on my Débian during 4 hours. No way, it can't start. Other tool like dia are not made for UML, and that's why IMHO there are not user friendly as UML modeller.

Well, Umbrello files are save in XMI. It should be possible to share xmi files between all UML tools. But, this is only theory. In XMI there are only the modell data stored (which classes, use cases and so on). But the diagram informations are not stored in XMI (so things like where on the diagram is the class, which colour and so on). There is till now no standard how to exchange these information.

Most tools save these information in the XMI file in a special section. So to import a file to another tool you need a filter which could interpret this special section. So it is very clear, that Rose/Argo or some other tools don't provide such filters for import Umbrello XMI files.

This is of course a mess. But in the upcoming UML 2.0 standard, this problem will be solved. There, diagram information will be saved as SVG. But 2.0 isn't published yet...

At a first try Umbrello allowed me to conveniently draw a diagram and save it and that's better than other open source UML diagramming tools I'd tried previously. At last an opportunity for me to learn UML hands-on. Great work!

ArgoUML is a nice tool but it's a little slow and the user interface is ugly. Poseidon is based on ArgoUML and has a cleaner user interface. Speed is about the same but maybe a bit faster. It's nice to be able to export diagrams as PNG. However, both these tools do not currently support PHP code generation to my knowledge. I know Umbrello does and this is why I'm considering trying it out. Oh yeah, one drawback to Poseidon is that it's not open source. Hence, the reason I don't want to use it. Anyway, I'm anticipating Umbrello 1.2 because it's going to include the missing deployment and component diagrams, will have bug fixes and will support code generation for many more languages. I hope more people contribute to Umbrello. We need a good enterprise UML tool that is open source and can be used to foster enterprise OSS development. It would also be nice to see Umbrello ported to different platforms to encourage its widespread use. But it's KDE-based so I don't know how much work that would take.

Does anyone know of any other UML tools that are open source and can generate PHP code?

I am now using Visual Paradigm for UML and Visual Paradigm SDE for NetBeans Community Edition. It is extermly fast. The Community Edition only support real-time Java code generation. I know the Professional Edition can do code generation better. I don't know is it supports PHP. But Visual Paradigm for UML is not open source.